On Fri, Aug 16, 2019 at 12:24:04PM -0400, Joel Fernandes wrote:
> On Thu, Aug 15, 2019 at 10:53:11PM -0400, Joel Fernandes (Google) wrote:
> > I really cannot explain this patch, but without it, the "else if" block
> > just doesn't execute thus causing the tick's dep mask to not be set and
> > causes the tick to be turned off.
> >
> > I tried various _ONCE() macros but the only thing that works is this
> > patch.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> > kernel/rcu/tree.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 856d3c9f1955..ac6bcf7614d7 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -802,6 +802,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
> > {
> > struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > long incby = 2;
> > + int dnn = rdp->dynticks_nmi_nesting;
>
> I believe the accidental sign extension / conversion from long to int was
> giving me an illusion since things started working well. Changing the 'int
> dnn' to 'long dnn' gives similar behavior as without this patch! At least I
> know now. Please feel free to ignore this particular RFC patch while I debug
> this more (over the weekend or early next week). The first 2 patches are
> good, just ignore this one.
Ah, good point on the type! So you were ending up with zero due to the
low-order 32 bits of DYNTICK_IRQ_NONIDLE being zero, correct? If so,
the "!rdp->dynticks_nmi_nesting" instead needs to be something like
"rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE", which sounds like
it is actually worse then the earlier comparison against the constant 2.
Sounds like I should revert the -rcu commit 805a16eaefc3 ("rcu: Force
nohz_full tick on upon irq enter instead of exit").
Or would that once again cause RCU to fail to enable the tick?
Thanx, Paul
> thanks,
>
> - Joel
>
>
> >
> > /* Complain about underflow. */
> > WARN_ON_ONCE(rdp->dynticks_nmi_nesting < 0);
> > @@ -826,7 +827,7 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
> >
> > incby = 1;
> > } else if (tick_nohz_full_cpu(rdp->cpu) &&
> > - !rdp->dynticks_nmi_nesting &&
> > + !dnn &&
> > rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> > rdp->rcu_forced_tick = true;
> > tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
> > --
> > 2.23.0.rc1.153.gdeed80330f-goog
> >
>
